I don't think
powders are problematic simply because they are powders, nor liquid detergents necessarily safer for being already in solution.
The real problems revolve around the contents of each formulation.
Until 2001, Electrolux used aluminum spiders which were subject to rapid decay when exposed to frequent use of LCB. They've stated this, it is beyond dispute. Since so many FLs in the US are from that manufacturer - and they otherwise have very long lives - I think this problem has gotten a lot of attention. I know many UK members here bleach with LCB but most Europeans either never use it or only very seldom, which is why Electrolux had no problem selling millions of these FLs over decades with such spiders.
Any product which is high sudsing will, eventually, cause problems in a FL because the rear bearing/s will fail. This, too, is independent of whether one uses powder or bleach.
A detergent which dissolves completely, rinses out completely, is low sudsing and has agents to protect against damage to the washer's components (meta-silicates, for instance) can't possibly be dangerous to the machine, regardless of whether liquid or powder.
That said, until liquid detergents are offered with effective low-temperature oxygen bleach, I'll stick with powders.
